Experimental study on ZnO-TiO<Subscript>2</Subscript> sorbents for the removal of elemental mercury

ZnO-TiO₂ sorbents synthesized by an impregnation method were characterized through XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy) and EDS (Energy dispersive spectrometer) analyses. An experiment concerning the adsorption of Hg⁰ by ZnO-TiO₂ under a simulated fuel gas atmosphere was then conducted in a bench-scale fixed-bed reactor. The effects of ZnO loading amounts and reaction temperatures on Hg⁰ removal performance were analyzed. The results showed that ZnO-TiO₂ sorbents exhibited excellent Hg⁰ removal capacity in the presence of H₂S at 150 °C and 200 °C; 95.2% and 91.2% of Hg⁰ was removed, respectively, under the experimental conditions. There are two possible causes for the H₂S reacting on the surface of ZnO-TiO₂: (1) H₂S directly reacted with ZnO to form ZnS, (2) H₂S was oxidized to elemental sulfur (S _ad ) by means of active oxygen on the sorbent surface, and then S _ad provided active absorption sites for Hg⁰ to form HgS. This study identifies three reasons why higher temperatures limit mercury removal. First, the reaction between Hg⁰ and H₂S is inhibited at high temperatures. Second, HgS, as the resulting product in the reaction of mercury removal, becomes unstable at high temperatures. Third, the desulfurization reaction strengthens at higher temperatures, and it is likely that H₂S directly reacts with ZnO, thus decreasing the S _ad on the sorbent surfaces.     

The effect of HCl and H<Subscript>2</Subscript>O on the H<Subscript>2</Subscript>S removing capacities of Zn-Ti-based desulfurization sorbents promoted by cobalt and nickel oxide

The sulfur removing capacities of various Zn-Ti-based sorbents were investigated in the presence of H₂O and HCl at high-(sulfidation, 650 °C; regeneration, 800 °C) and medium-(sulfidation, 480 °C; regeneration, 580 °C) temperature conditions. The H₂O effect of all sorbents was not observed at high-temperature conditions. At mediumtemperature conditions, the reaction rate of ZT (Zn/Ti : 1.5) sorbent decreased with the level of H₂O concentration, while modified (ZTC, ZTN) sorbents were not affected by the water vapor. HCl vapor resulted in the deactivation of ZT sorbent with a cycle number at high-temperature due to the production of ZnCl₂ while the sulfur removing capacities of ZTC and ZTN sorbents were maintained during 4–5 cyclic tests. In the case of medium-temperature conditions, ZT sorbent was poisoned by HCl vapor while cobalt and nickel added to ZT sorbent played an important catalytic role to prevent from being poisoned by HCl due to providing heat, emitted when these additives quickly react with H₂S even at medium-temperature conditions, to the sorbents     

Microchannel Autothermal Reforming of Methane to Synthesis Gas

Autothermal reforming of methane to synthesis gas (CO and H₂) is studied in a microchannel reactor comprised of Pt- and Rh-based catalysts that are coated on opposite walls of the channel. The effects of operating parameters and microchannel catalyst configuration on methane conversion and CO selectivity are analyzed. The parameters considered are the residence time of the reactants (12.9–25.7 ms), reaction temperature (500–650 °C), molar steam-to-carbon (S/C = 0–3.0) and oxygen-to-carbon (O₂/C = 0.47–0.63) ratios at the inlet. Doubling the residence time leads to ca. 10 % increase in methane conversion, but has only a 4 % contribution to the CO selectivity. Higher O₂/C ratios improve extent of methane oxidation, but reduce selectivity due to CO₂ production. When the temperature is raised from 500 to 650 °C, conversion increases from 12.8 to 46.6 % and selectivity increases from 20.1 to 35.7 %. S/C ratio has the greatest effect on the outlet H₂/CO ratio, which is found to vary between 0.93 and 2.68, via the water–gas shift reaction. Comparison of the present catalyst configuration with the use of bimetallic Pt–Rh coating in the microchannel under identical conditions shows that the latter can improve conversion by 20 % and CO selectivity by 33 %.     

Kinetic study of domestic limestone (Daesung)

Sulfidation and sulfation reactions of Daesung limestone, which is a calcium-based sorbent chosen out of domestic limestone for the removal of H₂S and SO₂, were investigated by using TGA (thermal gravimetric analyzer) Effects of H and H₂S on the sulfidation were also investigated The conversion rate of CaS to CaSO₄ in oxidation was low since the concentration of SO₂ used for this study was low and CaO was not completely converted into CaS It was observed that the effects of H₂ concentration on the sulfidation were relatively small and the maximum conversion rate and reaction rate increased with increase of H₂S concentration In the sulfation reaction, conversion rate could be raised with the injection of air at a sulfation reaction temperature above 800 °C However, the conversion rate decreased without the injection of air due to the blockage of sorbent pores.     

Correlation of H<Subscript>2</Subscript>S and COS in the hot coal gas stream and its importance for high temperature desulfurization

Thermodynamic analysis of the correlation of H₂S and COS has been carried out at the temperature range of 400–650 °C at which high temperature desulfurization of coal gas is usually performed. The correlation of the two sulfur species is mainly through the reaction H₂S+CO→COS+H₂. Simulated coal gas with the following composition CO 32.69%, H₂ 39.58%, CO₂ 18.27%, N₂ 8.92% and H₂S 0.47% was used in this study, and the equilibrium concentrations of the two species at different temperatures were calculated. The results of Fe-based sorbents during sulfidation were compared with calculations. It is concluded that the above reaction may reach equilibrium concentration in the presence of the Fe-based sorbents, which means the Fe-based sorbents may effectively catalyze the reaction between H₂S and CO. Because of the correlation of the two sulfur species, both can be effectively removed at high temperatures simultaneously, offering high temperature desulfurization some advantages over low temperature desulfurization processes.     

Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H2S

Removal of Hg⁰ vapor from the simulated coal combustion flue gases with a commercial activated carbon was investigated using H₂S. This method is based on the reaction of H₂S and Hg over the adsorbents. The Hg⁰ removal experiments were carried out in a conventional flow type packed bed reactor system in the temperature range of 80–150 °C using simulated flue gases having the composition of Hg⁰ (4.9 ppb), H₂S (0–20 ppm), SO₂ (0–487 ppm), CO₂ (10%), H₂O (0–15%), O₂ (0–5%), N₂ (balance gas). The following results were obtained: in the presence of both H₂S and SO₂, Hg removal was favored at lower temperatures (80–100 °C). At 150 °C, presence of O₂ was indispensable for Hg⁰ removal from H₂S–SO₂ flue gas system. It is suggested that the partial oxidation of H₂S with O₂ to elemental sulfur (H₂S+1/2O₂=S_ad+H₂O) and the Clause reaction (SO₂+2H₂S=3S_ad+2H₂O) may contribute to the Hg⁰ removal over activated carbon by the following reaction: S_ad+Hg=HgS. The formation of elemental sulfur on the activated carbon was confirmed by a visual observation.     

Sulphur capture during gasification of oil sand cokes

The capture of sulphur via the H₂S-CaO reaction during gasification of delayed coke has been investigated in a 6.4 cm dia. semi-batch stirred bed reactor. Calcines from one of three limestones or a dolomite were mixed with coke which was then gasified in a steam-nitrogen mixture at atmospheric pressure and 930°C. The H₂S content of the gas, and the sulphur content of the sorbent were determined as functions of time. The effects of sorbent surface area, percentage calcination, and the Ca/S ratio on the extent of sulphur capture and the conversion of sorbents were determined. The sulphidation reaction was analyzed using continuous and grain models. After an initial stage of chemical control the reaction appears to be controlled by diffusion through the product layer of the grain.     

An Unexpected Pressure Effect in the Catalytic Hydrogenolysis of a Complex Benzyl Amine

The selective hydrogenolysis of a complex benzylamine containing a pyridone ring and aryl fluorides was investigated. The aim of the research was to find a catalyst and reaction conditions under which the complete hydrogenolysis of the benzylamine could be accomplished with no aryl fluoride cleavage nor pyridone hydrogenation observed. This objective was accomplished by use of a specific 10 % Pd/C catalyst in a THF/EtOH solvent at 40 °C. It was found that as the H₂ pressure decreased the amount of debenzylated amine increased because of the decrease in pyridone hydrogenation. Unexpectedly, though, the rate of hydrogenolysis also increased with decreasing H₂ pressure. A rationale for this observation is presented.     

Removal of sulfur at high temperatures using iron-based sorbents supported on fine coal ash

This paper deals with the simultaneous removal of H₂S and COS in the temperature range of 400-650 °C at 1 bar by using iron-based sorbents. The iron-based sorbents were prepared using iron oxide and cerium oxide with coal fine ash as the support. Simulated coal gas was used in the sulfidation experiments and 5% O₂ in N₂ gas was used for regeneration of sorbents. Both sulfidation and regeneration experiments have been carried out using a fixed-bed quartz reactor. The product gases were analyzed using a GC equipped with a TCD and a FPD. The results demonstrated that both H₂S and COS can be effectively reduced using the iron-based sorbents supported on fine coal ash. XRD analysis shows that Fe_1−xS phase has formed during sulfidation indicating a high sulfur capacity of the sorbent. The mechanism of the removal of COS simultaneously with H₂S is also discussed.     

Supercritical water oxidation of quinoline in a continuous plug flow reactor—part 1: effect of key operating parameters

This work presents the supercritical water oxidation (SCWO) of quinoline, a nitrogen‐containing organic compound found in pharmaceutical wastewaters, to products that are more readily biodegradable. The effects of the operating variables, namely process temperature, stoichiometric ratio of oxidant to organic, residence time and system pressure were studied in order to optimise quinoline removal efficiency and to investigate the fate of carbon and nitrogen after oxidation. Hydrolysis experiments undertaken in the range 480–650 °C showed no significant degradation of quinoline. The present study confirmed that, as an alternative to incineration, the process is fast and effective in treating quinoline in water, converting it into primarily carbon dioxide (CO₂), water (H₂O) and nitrogen gas (N₂) and to a lesser extent ammonium ions (NH₄⁺). Temperature was shown to be the primary variable in the complete destruction of quinoline and TOC reduction, upon operating at around 250 bars. Essentially, complete quinoline removal was observed above 575 °C and complete TOC removal was obtained at 650 °C, where CO₂ was the main reaction product. Copyright © 2006 Society of Chemical Industry     

Comparison of CuO-MO x (M=Ce,Zn, Cr and Zr) catalysts in various water-gas shift reactions

The water-gas shift (WGS) reaction in the temperature range of 100–350 °C for various feed compositions simulating forward, reverse and real WGS conditions was studied for a series of coprecipitated mixed metal oxide catalysts of 30 wt% of CuO and 70 wt% of metal oxide (CeO₂, ZnO, Cr₂O₃, and ZrO₂) as well as for a commercial WGS catalyst (ICI 83-3). The catalysts were characterized using BET, XRD, H₂-TPR and N₂O dissociation studies. Among the tested catalysts, CuO-Cr₂O₃ showed the best activity in the forward WGS, while the commercial catalyst was the best catalyst in the real and reverse WGS reactions. The effect of Cu content in the catalyst was also studied and, in the case of the real WGS, 50 wt% CuO-Cr₂O₃ was more active than 30 wt% CuO-Cr₂O₃. H₂ and CO₂ were found to inhibit the forward WGS, decreasing the reaction rate substantially, particularly at temperatures below 200 °C. The inhibition effect varied depending on the tested catalyst and increased with increasing H₂ or CO₂ concentration. As the inhibition effect was reversible, the competitive adsorption of H₂ or CO₂ on the active sites has been suggested to be responsible for the effect. The high activity of the commercial catalyst in the H₂ rich real WGS could be described by the difference in the H₂ inhibition between the catalysts. An easily reducible copper species was found in CuO-Cr₂O₃ and could be attributed to the high activity in the forward WGS. Keywords:Mixed Metal Oxide Catalysts, Water Gas Shift Reaction, H₂ Inhibition, CO₂ Inhibition, Copper Chromate Catalyst     

Effect of water pretreatment on CO<Subscript>2</Subscript> capture using a potassium-based solid sorbent in a bubbling fluidized bed reactor

A bubbling fluidized bed reactor was used to study CO₂ capture from flue gas by using a potassium-based solid sorbent, sorbKX35 which was manufactured by the Korea Electric Power Research Institute. A dry sorbent, sorbKX35, consists of K₂CO₃ for absorption and supporters for mechanical strength. To increase initial CO₂ removal, some amount of H₂O was absorbed in the sorbent before injecting simulated flue gas. It was possible to achieve 100% CO₂ removal for more than 10 minutes at 60°C and a residence time of 2 s with H₂O pretreatment. When H₂O pretreatment time was long enough to convert K₂CO₃ of sorbKX35 into K₂CO₃ · 1.5H₂O, CO₂ removal was excellent. The results obtained in this study can be used as basic data for designing and operating a large scale CO₂ capture process with two fluidized bed reactors. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

The removal of hydrogen sulfide with manganic sorbent in a high-temperature fluidized-bed reactor

The removal of hydrogen sulfide (H₂S) from simulated gas was carried out in a batch type fluidized-bed reactor using natural manganese ore (NMO), which consists of several metal oxides (MnO_x: 51.85%, FeO_y: 3.86%, CaO: 0.11%). The H₂S breakthrough curves were obtained by changing temperature, gas velocity, initial H₂S concentration, and aspect ratio. Moreover, the effects of the particle size and the particle-mixing fraction on H₂S removal were investigated in a binary system of different particle size. From this study, H₂S removal efficiency increased with increasing temperature but decreased with increasing excess gas velocity. The breakthrough time for H₂S decreased as the gas velocity increased, which leads to reducing gas-solid contacting due to gas bypassing in a fluidized bed reactor. Improvement of H₂S removal efficiency in continuous process can be expected from the results of the binary particle system with different size in a batch experiment. The NMO could be considered as a potential sorbent in H₂S removal.     

Regenerable Sorbents for High-Temperature Desulfurization of Syngas from Biomass Gasification

Single-metal high-temperature solid sorbents for syngas cleaning using Mn, Ca, Fe, Cu, or Mo supported on γ-Al₂O₃ were synthesized, characterized, and tested in a fixed-bed reactor. H₂S and SO₂ concentrations in the gas after treatment at T = 400 to 700 °C were compared with thermodynamic calculations. The Mn-based sorbent showed the best ability to achieve a low sulfur residual in the gas, especially at temperatures above 600 °C. Sorbents with Fe, Cu, and Mo gave SO₂ formation in the initial phase, but this could be avoided by a pre-reduction treatment of the sorbent material.     

Brown coal conversion under the action of supercritical water

A test bench was developed and the conversion of the organic matter of coal (OMC) in supercritical water (SCW) was studied under conditions of a continuous supply of a water-coal suspension to a vertical flow reactor at 390–760°C and a pressure of 30 MPa. From 44 to 63% OMC was released as liquid and gaseous products from coal particles (from the water-coal supension) during the time of fall to the reactor. This stage was referred to as the dynamic conversion of coal. The particles passed through the stage of the dynamic conversion of coal did not agglomerate in the reactor in the subsequent process of batch conversion in a coal layer at T = 550–760°C. The volatile products of the overall process of the dynamic and batch conversion of coal included saturated hydrocarbons (CH₄ and C₂H₆), aromatic hydrocarbons (C₆H₆, C₇H₈, and C₈H₁₀), synthesis gas (H₂ and CO), and CO₂. At T < 600°C, CO₂ and CO were the degradation products of oxygen-containing OMC fragments, whereas they also resulted from the decomposition of water molecules at higher temperatures in accordance with the reaction (C) + H₂O = CO + H₂. The mechanisms were considered, and the parameters responsible for the dynamic conversion of coal were calculated.     

Effect of heat treatment on surface structure and gas sensing of electrospun ZnO-SnO2 composite nanofibers

ZnO-SnO₂ composite nanofibers with a constant Zn/Sn ratio of 0.4 have been electrospun and calcined at 650°C in ambient air, followed then by heat treatment at 350°C in either air, 5% H₂-95% N₂, or 15 ppm H₂S-air atmosphere for comparison of gas-sensing behaviors. The nanofibers being heat-treated in the H₂S-air atmosphere display a sensing response more than 25 times than the as-calcined counterpart, that is, the sensing response increases from 20 to 514 against a model NO₂ gas of 10 ppm concentration at a working temperature of 180°C. This appears to be attributable to the formation of sulfate on the nanofiber surface, which resulted in an enriched oxygen vacancy and chemisorbed oxygen near the surface for facilitating the redox reaction toward NO₂ gas molecules. The facile heat treatment in the presence of dilute H₂S concentration may have opened up an alternative route for enhancing the surface-sensitive gas-sensor activity.     

Henry's law constant for hydrogen in high temperature water with dissolved lithium and boron

The results of measurement of the Henry's law constant are reported for solutions of H₂ in H₂O, with dissolved lithium and boron, at high temperature (220 to ～300°C). The experimental procedure involved the use of a palladium/hydrogen electrical resistance sensor to determine the hydrogen gas pressure in equilibrium with the hydrogen dissolved in the water. Values of for the above solutions were lower than values for the solute‐free H₂/H₂O solutions. For conditions typical of a Pressurized Water Reactor (PWR) the lowering amounted to about 3%.     

Characteristics of the removal of mercury vapor in coal derived fuel gas over iron oxide sorbents

The characteristics of a novel method for Hg removal using H₂S and sorbents containing iron oxide were studied. Previously, we have suggested that this method is based on the reaction of Hg and H₂S over the sorbents to form HgS. However, the reaction mechanism is not well understood. In this work, the characteristics of the Hg removal were studied to clarify the reaction mechanism. In laboratory made sorbents containing iron oxide were used as the sorbent to remove mercury vapor from simulated coal derived fuel gases having a composition of Hg (4.8 ppb), H₂S (400 ppm), CO (30%), H₂ (20%), H₂O (8%), and N₂ (balance gas). The following results were obtained: (1) The presence of H₂S was indispensable for the removal of Hg from coal derived fuel gas; (2) Hg was removed effectively by the sorbents containing iron oxide in the temperature range of 60-100 °C; (3) The presence of H₂O suppressed the Hg removal activity; (4) The presence of oxygen may play very important role in the Hg removal and; (5) Formation of elemental sulfur was observed upon heating of the used sample.     

Oxidative regeneration of sulfided sorbent by H2S without emission of SO2

Much SO₂, another perilous air pollutant, was emitted during the oxidative regeneration of sulfided sorbent by H₂S. In order to prevent emission of SO₂, we carried out oxidative regeneration with the physical mixture of CaO and sulfided sorbent and investigated the effect of regeneration temperature and oxygen concentration on the reactivity of CaO with S0₂. The effluent gases were analyzed by G.C. and the properties of sorbent were characterized by XRD. SEM, TG/DTA and EPMA. Deterioration of reactivity of CaO with S0₂ resulted in increment of emission of SO₁₂ due to the structural changes of CaO above 750°C and that at 850°C was more severe. Furthermore EPMA and XRD analysis revealed that product layer diffusion through the solid product, CaSO₄, was the rate limiting step for CaO sulfidation. The reaction of CaO w:.th SO₂ was first order approximately and that was accelerated by high O₂ concentration.     

Elemental sulfur production through regeneration of a SO2-adsorbed V2O5-CoO/AC in H2

H₂ regeneration of an activated carbon supported vanadium and cobalt oxides (V₂O₅-CoO/AC) catalyst–sorbent used for flue gas SO₂ removal is studied in this paper. Elemental sulfur is produced during the H₂-regeneration when effluent gas of the regeneration is recycled back to the reactor. The regeneration conditions affect the regeneration efficiency and the elemental sulfur yield. The regeneration efficiency is the highest at 330 °C, with SO₂ as the product. The production of elemental sulfur occurs at 350 °C and higher with the highest elemental sulfur yield of 9.8 mg-S/g-Cat. at 380 °C. A lower effluent gas recycle rate is beneficial to elemental sulfur production. Intermittent H₂ feeding strategy can be used to control H₂S concentration in the gas phase and increase the elemental sulfur yield. Two types of reactions occur in the regeneration, reduction of sulfuric acid to SO₂ by AC and reduction of SO₂ to elemental sulfur through Claus reaction. H₂S is an intermediate, which is important for elemental sulfur formation and for conversion of CoO to CoS that catalyzes the Claus reaction. The catalyst–sorbent exhibits good stability in SO₂ removal capacity and in elemental sulfur yield.